For various reasons, a wafer to be electroplated may be tilted to a non-horizontal angle upon immersion into an electroplating bath. Thus, some existing methods and apparatus for electroplating require that a substrate be immersed in plating solution over a considerable period of time (e.g., 120-200 ms from the time the leading edge enters solution to the time the trailing edge is fully immersed) as compared to the total time it takes to fill features on a substrate (e.g., current technology node wafer structures may fill in about 1-2 s, in some cases in less than about 500 ms). The relatively long immersion time (defined as the time it takes for the entire plating face of the substrate to become immersed in plating solution) results in inconsistent feature fill because the leading edge of the substrate enters the plating solution and begins plating before the trailing edge of the substrate. Initial plating non-uniformities may persist throughout the plating process, resulting in non-uniform fill. These effects will be further exacerbated as the industry moves from 300 mm to 450 mm wafers. Without wishing to be bound by any theory or mechanism of action, the difference in plating start time across the wafer may lead to non-uniform adsorption of additives such as accelerators, suppressors and levelers, which can lead to non-uniform plating across the surface of the wafer. Therefore, it is generally better to have a short immersion time relative to the time it takes to fill small features, so that the difference in fill start time across the wafer can have minimal impact such that feature fill and plating uniformity can be maximized.
One consideration in minimizing immersion time is the formation of bubbles at the interface between the plating solution and the substrate. During wafer immersion into a plating electrolyte, bubbles can be entrapped under the plating side of the wafer (the active side or plating surface). The bubble trapping issue may be exacerbated if the substrate is immersed too quickly. Air bubbles trapped on the plating surface of a wafer can cause many problems. Where bubbles are present, they shield the plating surface of a wafer from exposure to electrolyte, and thus produce a region where plating does not occur. The resulting plating defect can manifest itself as a region of no plating or of reduced plating thickness, depending on the time at which the bubble became entrapped on the wafer and the length of time that it stayed entrapped there. Thus, under current electroplating methods, significant plating defects may occur if the immersion time is too fast.